(1) Field of the Invention
The present invention relates to key telephones and more particularly to an electronic exclusion circuit for use in said key telephones. This exclusion circuit provides an exclusion release feature operated in response to operation of an exclusion release key or partial operation of a hold key.
(2) Description of the Prior Art
Automatic exclusion circuits are old and well known and operate to exclude telephones from busy lines. Such a circuit is shown in U.S. Pat. Nos. 3,961,144 and 4,021,621.
An exclusion release feature is also known in the prior art whereby a telephone connected to a telephone line can release an excluded telephone, thereby allowing that telephone to connect to the busy line. Such a circuit is shown in U.S. Pat. Nos. 4,053,720 and 4,117,274. Both of these circuits require operation of an exclusion release key to provide this feature.
The exclusion release function can also be performed by partial operation of the hold key as disclosed in GTE Automatic Electric Incorporated circuit WW-14615.
However, these prior art exclusion release circuits do not provide the exclusion release function through both partial operation of a hold key and operation of an exclusion release key. Furthermore, none of these circuits provide a visual indication of the excluded state of a key telephone or operation of the exclusion release function of a key telephone.
GTE Automatic Electric Incorporated circuit, HH-880054-1 provides these features, however, it is implemented by means of relay logic. These components create cost and reliability problems.
Accordingly, it is the object of the present invention to provide a novel, low cost and highly reliable exclusion release circuit.
A feature of the present invention is an exclusion release function controlled by either partial operation of a hold key or operation of a dedicated exclusion release key.
Another feature of the present invention is provision of a visual indication of the excluded state of a key telephone and the operation of the exclusion release function.